The yellow fever virus [YFV] 17D vaccine strain is one of the most effective and safe vaccines available. Immunization with YF-17D induces a long-term neutralizing antibody response and strong CTL activation, and provides excellent protection against infection with virulent YFV. The overall goal of the present application is to understand the immunologic and genomic pathways which regulate the immune response to YF-17D, with a view to targetting such pathways to enhance the sub-optimal immune responses triggered by certain subunit vaccines such as the anthrax vaccine [AVA], In this context, the present proposal will be driven by the following hypotheses: (i) The strength, quality and duration of a vaccine is critically dependent on its effects on innate immune activation. Thus YF-17D strongly activates innate immune and dendritic cell [DC] responses, while AVA is poorly stimulatory (ii) Local injections of a strong vaccine, such as YF-17D will stimulate "early global responses," in the blood, characterized by systemic DC activation and rapid changes in the transcriptome and proteome of blood immunocytes (iii) Such early global responses may contain molecular signatures, which could be used to predict the strength, quality and duration of the adaptive immunue response that follows (iv) The weak immunogenicity of AVA, may in part, be attributed to the induction of regulatory T cells. Such regulatory T cells may be induced by anthrax toxin, lethal factor, a likely contaminant in AVA, which impairs antigen-presentation and adaptive immunity, via suppression of the MAP-kinase activity in DCs (v) The weak immunogenicity of AVA can be significantly augmented by triggering particular TLRs on DCs. Such TLR signaling will augment DC function and immune response and suppress the development of regulatory T cells. These hypotheses will be tested in the following Specific Aims: AIM 1: To determine the early innate and DC responses in vivo, in humans vaccinated with YF-17D or AVA; AIM 2: To characterize the in vitro response of human DC subsets to YF 17D, AVA or AVA + TLR ligands; Sub-Aim 2a: To characterize the in vitro response of human DC subsets to YF-17D and AVA. Sub-Aim 2b: To determine whether the in vitro DC response to AVA can be enhanced by triggering TLRs on DCs; Aim 3: To correlate the early innate and DC responses, with the adaptive immune responses stimulated in vivo by YF-17D, AVA, or AVA + TLR ligands in non-human primates; Sub-Aim 3a: Analyses of early innate and DC responses in the blood and lymph nodes; Sub-Aim 3b: Analyses of antigen-specific CD4+ and CD8+ T cell responses. In summary, the present proposal aims to understand the immune regulatory mechanisms of one of the most effective vaccines, and then to use such mechanisms to augment the potency of weaker vaccines.